The present invention relates to a process for the production of dithionites.
It has been known that anhydrous dithionites can be prepared by a process which comprises reacting formic acid or a formate, alkali compounds, and sulfurous acid anhydride in a mixture of water and organic solvent. In this process, the dithionites precipitate in the reaction solution as the reaction proceeds and, therefore, after the reaction is completed, the dithionites thus precipitated can be separated from the mother liquor by filtration to obtain crystals of the dithionites.
If the thus-obtained crystals are dried, the purity of the dithionite product is low because impurities contained in the mother liquor remain on and/or in the crystals. Furthermore, the water contained in the mother liquor seriously retards drying and accelerates decomposition of the dithionites. This decomposition of the dithionites is very dangerous. At high rates of decomposition, the dithionites may catch fire in the drying apparatus. Usually, therefore, it is essential that the crystals of the dithionites separated from the mother liquor by filtration are fully washed with an organic solvent, e.g. methanol or ethanol, prior to being dried to remove the mother liquor remaining on and/or in the crystals which contains water and salt impurities.
Techniques for washing dithionites have rarely been reported and it has been merely reported that dithionite crystals are washed with an organic solvent after the filtration thereof. In general, the washing liquid used is purified by distillation, for example, either independently or in combination with the mother liquor to recover the organic solvent. For example, Japanese patent publication No. 38556/1973 discloses that alcohols used in the reaction can be recovered by distillation and reused. This distillation, however, requires a great amount of energy. It is necessary from the view point of energy saving to solve such a problem.
When a solution containing thiosulfates, resulting from the reaction of synthesizing dithionites, is recycled and reused in the reaction, the thiosulfates have an adverse influence on the reaction. This gives rise to the problems that the reaction of synthesizing the dithionites is inhibited and the decomposition of the resulting dithionites is undesirably accelerated.